1. Field of the Invention
The present invention relates to a secondary battery, and more particularly to a lithium ion/polymer secondary battery.
2. Description of the Related Art
Generally, secondary batteries are rechargeable batteries, differently from primary batteries, and such secondary batteries are widely used as power sources of electric vehicles or electronic devices such as cellular phones, notebooks and camcorders. In particular, a capacity of a lithium secondary battery with an operation voltage of 3.6V is about three times or more of that of nickel-cadmium batteries or nickel-hydrogen batteries frequently used as power source of electronic devices. In addition, the lithium secondary battery has a high energy density per unit weight, so lithium secondary batteries tend to be more increasingly used.
The lithium secondary battery frequently uses lithium-based oxides and carbon materials as positive active material and negative active material, respectively. Also, the lithium secondary battery may be classified into rectangular cells, cylindrical cells and pouch-type cells.
The lithium ion secondary battery includes an electrode assembly in which a positive electrode, a separator and a negative electrode are arranged in order, and an enclosure sealing the electrode assembly together with electrolyte. In particular, an enclosure of a rectangular-type or cylindrical-type secondary battery includes a case having an open end and a cap assembly sealing the open end of the case.
The electrode assembly is classified into a jelly-roll type electrode assembly in which a separator is interposed between sheet-type positive and negative electrodes respectively coated with active materials and then rolled, and a stack-type electrode assembly in which a separator is interposed between positive and negative electrodes of a predetermined size such that they are stacked in order. Here, the jelly-roll type electrode assembly is widely used since the jelly-roll type electrode assembly is advantageous in easy production and high energy density per weight and is easily received in a cylindrical or rectangular case. Meanwhile, the stack-type electrode assembly is widely used as pouch-type batteries.
However, when a secondary battery is charged/discharged, the electrode assembly tends to be deformed through repeated expansion and shrinkage, and in this procedure, in case of the jelly-roll type electrode assembly, stress is focused on a metallic center pin, so electrodes may pierce into the separator and come into contact with the metallic center pin, which causes internal electric short circuit. Such an internal short circuit of the secondary battery makes the battery generate heat, and the heat may decompose organic solvent to generate gas, thereby increasing the pressure in the battery and rupturing the enclosure. The gas pressure in the battery may also be increased due to internal short circuit caused by an external impact.
In order to solve such a safety-related problem of a battery, the secondary battery is basically provided with a PTC element. In particular, a cylindrical secondary battery includes a cap assembly that includes safety devices such as a safety vent for discharging a high-pressure gas and a CID (Current Interrupt Device) for interrupting an electric current when an inner pressure of the battery is increased, and a top cap for forming a protruded terminal protecting such safety devices. Also, the cap assembly is sealed with the case by means of a gasket.
However, in such a conventional secondary battery, while the cap assembly is assembled with the case by means of the gasket, the possibility of creation of a gap is very high between the gasket and the cap assembly or between the gasket and the case. Such a gap deteriorates sealing of the battery. In other words, the gasket is compressed and deformed due to the case during a clamping process or the like, so the case is closely adhered to the cap assembly. However, in the gasket adopting a conventional structure, a surface closely adhered between the case and the cap assembly has a simple flat shape, so its sealing ability is not so good. In particular, if the clamping surface of the gasket is not uniformly pressed while the case is clamped, the flat surface of the gasket is deformed unevenly, so the gasket is partially not adhered to the case or the cap assembly, thereby creating a gap and thus deteriorating the sealing with the case.
In order to solve this problem, Korean Patent Publication No. 10-2006-0037595 discloses a secondary battery having at least one wrinkle with an uneven shape formed at a surface of a gasket, which is contacted with a case and a cap assembly. However, this secondary battery has an uneven structure only at the gasket made of plastic, so the deterioration of sealing of the secondary battery is still not overcome. The sealing structure of such a gasket of the conventional secondary battery is designed for sealing the interface portion between the safety vent located at a primarily exposed portion of electrolyte and/or gas, or a lowermost layer of the cap assembly, and the gasket surrounding an outer periphery of the safety vent, so the above problem occurs. In other words, in the sealing manner disclosed in the conventional document, in a state that electrolyte or the like is leaked through the interface between the safety vent and the gasket surrounding the outer periphery of the safety vent, a final end of the interface is sealed. Thus, the sealing structure between the cap assembly and the gasket according to the conventional technique has an obvious limit.